We have a variety of breeds in our flock and some of them have started laying. We now have about 24 eggs right now but some of them are speckled eggs (See picture). We have a Speckled Sussex, a Columbian Rock, a few Barred Rocks, a few Buff Brahmas, a Red Rock cross, and a Partridge Wyanndotte.

The bottom egg is speckled compared to the egg on the top of the picture.

All of those breeds lay brown eggs, so it could come from any of them, especially if you got them from a feed store. I've had chickens who are the same breed lay different shades of brown before. The only way to know who is laying it would be to watch them lay it or lock all your birds in different cages.